javascript - Can't find variable letter in React Native setState - Stack Overflow

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I tried using setState in a TextInput like so:

class AppS extends Component {
  constructor(props){
    super(props);
    this.state = { 
      ShowText: '',
      letter: ''
    };
  }
  render(){
    return( 
      <View>  
        <TextInput style ={{color: "green"}}
          onChangeText = {(letter)=>this.setState({ letter })}>
        </TextInput>
        <Text style = {styleForApps.fon}> {this.state.letter} </Text>
        <Text style = {styleForApps.body}> {this.state.ShowText} </Text>
      </View>
    );
  }
}

When I experiment with setState like this:

onChangeText = {(txt)=>this.setState({ letter: txt })}>

It works well. But when I changed it to:

onChangeText = {(txt)=>this.setState({ letter })}> 

It always give a error "Can't find variable letter". I thought that setState point the parameter in parenthesis that should be defined by txt.

Can you explain please, I don't understand how exactly it passes the value to letter.

I tried using setState in a TextInput like so:

class AppS extends Component {
  constructor(props){
    super(props);
    this.state = { 
      ShowText: '',
      letter: ''
    };
  }
  render(){
    return( 
      <View>  
        <TextInput style ={{color: "green"}}
          onChangeText = {(letter)=>this.setState({ letter })}>
        </TextInput>
        <Text style = {styleForApps.fon}> {this.state.letter} </Text>
        <Text style = {styleForApps.body}> {this.state.ShowText} </Text>
      </View>
    );
  }
}

When I experiment with setState like this:

onChangeText = {(txt)=>this.setState({ letter: txt })}>

It works well. But when I changed it to:

onChangeText = {(txt)=>this.setState({ letter })}> 

It always give a error "Can't find variable letter". I thought that setState point the parameter in parenthesis that should be defined by txt.

Can you explain please, I don't understand how exactly it passes the value to letter.

• javascript
• reactjs
• react-native
• ecmascript-6

Share Improve this question Follow edited Oct 11, 2017 at 22:29 Andrew Li 58k1414 gold badges134134 silver badges148148 bronze badges asked Oct 11, 2017 at 22:07 GoldGold 1111 silver badge77 bronze badges

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2 Answers 2

Sorted by: Reset to default 7

You're attempting to use shorthand property definitions. This ES6 feature allows you to only specify the key if the key and value are the same. It does not take the value of the argument. That means when you do:

this.setState({
  letter
});

It means this:

this.setState({
  letter: letter
});

Because it desugars so that the key and value are the same. This then reports the error because letter isn't defined. You could do:

(letter) => this.setState({ letter })

This works because you name the argument letter, not txt.

onChangeText takes a function and passes the text that the user enters into that function, in your example you're naming that user input as txt but inside of setState you're not assigning this user input to letter, what you're doing is specifying this.setState({letter}) which gets translated into this.setState({letter: letter}) and because you don't have a variable named letter you get an error

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